Optical fibers serve as waveguides for optical energy and are useful for many applications. Being capable of transmitting signals at light speed, highly immune to noise and crosstalk, offering low loss, high bandwidth, and excellent electrical isolation, optical fibers frequently serve as signal transmission lines. Beyond serving basic transmission line functions, optical fibers can be specially formed in ways enabling them to serve more sophisticated functions. For example, special functions can be carried out by devices made by tapering one or more segments of a single or fused fiber.
Through tapering, single and/or fused optical fibers can be adapted to applications for which untapered fibers are unsuitable. Tapered fibers have been designed to serve as spectral filters, frequency shifters, switches, variable couplers, tunable and nontunable filters, resonators and more. A number of such devices have been described for example by T. A. Birks, P. S. Russell and D. O. Culverhouse, in “The Acousto-Optical Effect In Single Mode Fiber Tapers and Couplers”, Journal of Light Wave Technology, volume 14, number 11 pages 2519-2529, November 1996. Tapered optical fiber devices find application for example in the computer, telecommunications, aeronautics, and television broadcasting industries.
In an untapered optical fiber, the optical field is generally confined to a small region in the center of the fiber where it is guided by a central region referred to as the core. The core is encased in a surrounding material called cladding. The core has a somewhat higher index of refraction than the surrounding cladding. Currently, most fibers used for telecommunications applications are designed to support only a single guided mode. Tapering a single mode fiber causes the light to transition from the mode in the core of the untapered fiber to the lowest order mode of the taper waist. If tapering is sufficiently gradual, this transition can take place with very low optical loss. The optical energy in the tapered region is then guided by the boundary between the cladding and the material surrounding the cladding, often air. Tapering also allows access to the evanescent tail of the optical mode field as it extends out of the taper waist into the surrounding material. Two or more fibers may also be fused to form couplers or devices for performing other functions achieved through interaction of signals carried by more than one fiber.
The term “radial profile” (or equivalently “diametrical profile” due to fiber cross-sectional symmetry), refers to the shape according to the radius (or diameter) of a segment of a fiber that changes with distance over its length along the longitudinal axis of the segment. The shape and length of the radial profile are highly determinative of the characteristics and performance of tapered fiber optical devices. Radial profile has a crucial bearing on important properties of a fiber, including such properties as bend tolerance, number of modes supported and the effective index of refraction of the guided modes.
Devices have been demonstrated that rely on access to the evanescent field facilitated by fiber tapering. These include micro ring resonator filters fabricated from micro spheres, micro toroids and tapered fiber cylinders. Devices such as tunable phase shifters and attenuators are also possible by immersing the tapered fiber in a material with controllable refractive index or absorption.
Another class of devices that may be implemented with tapered optical fiber takes advantage of mode coupling between the multiple modes that may be supported by the taper waist. Optical notch filters may be fabricated by employing a periodic perturbation to the taper waist with a period chosen to match the beat length between modes at a particular resonant wavelength. One method of producing such a notch filter is to employ an ultrasonic transducer to launch an acoustic flexural wave into the taper waist. Filters of this type have been demonstrated that are electrically tunable in both resonant wavelength and notch depth. These acousto-optic filters take advantage of coupling between forward propagating modes in the coupler waist because the beat length between these modes is relatively long. It is also possible to produce devices that couple between forward and backward propagating modes in the taper waist. The beat length between these modes is on the order of one half of the optical wavelength. Perturbations with such short periods may be implemented in the material surrounding the fiber taper.
Yet another class of devices that may be implemented with tapered optical fiber takes advantage of the strong index guide that is created by the large index of refraction step between the tapered fiber and the surrounding air. Because of this large step in index, tight bends may be achieved with relatively low optical loss. Optical “turnarounds” have been commercialized that rely on this property to redirect light one hundred eighty degrees (180°) in very small packages.
Because the evanescent fields in a taper waist extend into the surrounding medium, tapered fibers also find application as sensing devices. Tapered optical fiber-based devices have been applied to biological sensing and chemical sensing. Tapered optical fibers have also been used as a key component in highly sensitive strain and pressure sensors, temperature sensors, and refractive index sensors.
Tapered optical fibers are also useful in laser applications. Tapered fibers are important for coupling from micro resonator lasers, and have been demonstrated as a platform for dye lasers.
A common requirement of many of the aforementioned devices and device applications is that the dimensions of the tapered fiber be tightly controlled. The effective index of the optical modes supported in the taper waist varies with the taper waist diameter. Thus, many device applications have strict requirements on radial profile shape and uniformity. In addition, the radial profile of tapered segments of optical fiber most often must be smoothly varying with axial length in order to minimize optical loss resulting from transitions in radial dimension.
Tapering involves drawing a segment of the fiber, or pair of fused fibers, into an elongated, reduced diameter portion, sometimes called a “waist”. A common method of fabricating tapered and fused fiber devices involves heating a fiber with a small, axially movable, flame to soften them sufficiently that one or both ends of the fiber can be pulled axially to draw the fiber into a taper. An example of such technique and an apparatus for carrying it out have been described by T. A. Birks and Y. W. Li, “The Shape of Fiber Tapers,” Journal of light wave technology, volume 10, no. 4 pp 432-438, April, 1992. That technique can also be used to concurrently taper and fuse to one another a pair of adjoining fibers.
Another method and apparatus for fabricating a fiber optic fused coupler is described for example in U.S. Pat. No. 5,931,983 to Bloom. According to that patent, a pair of adjoining optical fibers is heated and softened by means of a heat source located a predetermined distance from the fibers. The degree of coupling between the fibers is monitored by a laser device. Once a portion is softened, the unsoftened ends of the fiber are pulled apart axially at an initial pulling velocity. The heat source is moved away from the fibers and the pulling velocity selectively reduced in response to a substantial change in coupling ratio as detected by the laser device.
However, the capabilities of prior art fabrication methods and equipment have been limited in at least two significant respects. The complexity of the shapes of radial profiles which can be formed with suitable dimensional accuracy and repeatability has been very limited. In the prior art, only radial profiles of relatively simple shape could be formed reliably. For example, prior art methods and apparatus have been capable of forming axially symmetrical “hourglass” shaped tapers with mutually opposing transition portions whose diameters decrease monotonically and meet a thinned waist of substantially uniform diameter. Relatively simple radial profiles of substantially linearly or exponentially decreasing shape, or radial profiles controlled principally as to overall length and having a waist of nominally constant radius also have been obtained by linearly varying the length of the hot zone as elongation of the fiber proceeds such as by sweeping the heat source in an oscillatory manner as described by T. A. Birks and Y. W. Li, infra.
Prior art optical fiber tapering methods and apparatus have also been limited as to their ability to create optical fibers having desired radial profiles that: (i) can readily be specified in advance in an unambiguous way; (ii) suitably conform in both shape and dimension to the desired radial profile; (iii) are capable of exhibiting smooth axial transitions in radius; (iv) are readily reproducible with high repeatability, and can have shapes more complex than those which have been produceable heretofore.
Improving the performance of optical devices and systems will require the ability to fabricate tapered optical fibers with radial profiles that can be formed more predictably and with greater control over shape and dimensional accuracy than has been possible in the prior art. The development of new kinds of optical devices, capable of performing functions not previously possible, will demand the ability to fabricate tapered optical fibers having radial profiles of more complex shapes than those which can be realized using prior art tapering methods and apparatus.